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Pytest support for asyncio.

Project description

pytest-asyncio is an Apache2 licensed library, written in Python, for testing asyncio code with pytest.

asyncio code is usually written in the form of coroutines, which makes it slightly more difficult to test using normal testing tools. pytest-asyncio provides useful fixtures and markers to make testing easier.

async def test_some_asyncio_code():
    res = await library.do_something()
    assert b'expected result' == res

pytest-asyncio has been strongly influenced by pytest-tornado.


  • fixtures for creating and injecting versions of the asyncio event loop

  • fixtures for injecting unused tcp ports

  • pytest markers for treating tests as asyncio coroutines

  • easy testing with non-default event loops

  • support for async def fixtures and async generator fixtures


To install pytest-asyncio, simply:

$ pip install pytest-asyncio

This is enough for pytest to pick up pytest-asyncio.



Creates and injects a new instance of the default asyncio event loop. By default, the loop will be closed at the end of the test (i.e. the default fixture scope is function).

Note that just using the event_loop fixture won’t make your test function a coroutine. You’ll need to interact with the event loop directly, using methods like event_loop.run_until_complete. See the pytest.mark.asyncio marker for treating test functions like coroutines.

Simply using this fixture will not set the generated event loop as the default asyncio event loop, or change the asyncio event loop policy in any way. Use pytest.mark.asyncio for this purpose.

def test_http_client(event_loop):
    url = ''
    resp = event_loop.run_until_complete(http_client(url))
    assert b'HTTP/1.1 200 OK' in resp

This fixture can be easily overridden in any of the standard pytest locations (e.g. directly in the test file, or in to use a non-default event loop. This will take effect even if you’re using the pytest.mark.asyncio marker and not the event_loop fixture directly.

def event_loop():
    loop = MyCustomLoop()
    yield loop

If the pytest.mark.asyncio marker is applied, a pytest hook will ensure the produced loop is set as the default global loop. Fixtures depending on the event_loop fixture can expect the policy to be properly modified when they run.


The event_loop_process_pool fixture is almost identical to the event_loop fixture, except the created event loop will have a concurrent.futures.ProcessPoolExecutor set as the default executor.


Finds and yields a single unused TCP port on the localhost interface. Useful for binding temporary test servers.


A callable which returns a different unused TCP port each invocation. Useful when several unused TCP ports are required in a test.

def a_test(unused_tcp_port_factory):
    port1, port2 = unused_tcp_port_factory(), unused_tcp_port_factory()

Async fixtures

Asynchronous fixtures are defined just like ordinary pytest fixtures, except they should be coroutines or asynchronous generators.

async def async_gen_fixture():
    await asyncio.sleep(0.1)
    yield 'a value'

async def async_fixture():
    return await asyncio.sleep(0.1)

All scopes are supported, but if you use a non-function scope you will need to redefine the event_loop fixture to have the same or broader scope. Async fixtures need the event loop, and so must have the same or narrower scope than the event_loop fixture.

If you want to do this with Python 3.5, the yield statement must be replaced with await yield_() and the coroutine function must be decorated with @async_generator, like so:

from async_generator import yield_, async_generator

async def async_gen_fixture():
    await asyncio.sleep(0.1)
    await yield_('a value')



Mark your test coroutine with this marker and pytest will execute it as an asyncio task using the event loop provided by the event_loop fixture. See the introductory section for an example.

The event loop used can be overriden by overriding the event_loop fixture (see above).

In order to make your test code a little more concise, the pytest pytestmark feature can be used to mark entire modules or classes with this marker. Only test coroutines will be affected (by default, coroutines prefixed by test_), so, for example, fixtures are safe to define.

import asyncio
import pytest

# All test coroutines will be treated as marked.
pytestmark = pytest.mark.asyncio

async def test_example(event_loop):
    """No marker!"""
    await asyncio.sleep(0, loop=event_loop)


The asyncio_process_pool marker is almost identical to the asyncio marker, except the event loop used will have a concurrent.futures.ProcessPoolExecutor set as the default executor.


0.7.0 (2017-09-08)

  • Python versions pre-3.6 can use the async_generator library for async fixtures. #62 <>

0.6.0 (2017-05-28)

  • Support for Python versions pre-3.5 has been dropped.

  • pytestmark now works on both module and class level.

  • The forbid_global_loop parameter has been removed.

  • Support for async and async gen fixtures has been added. #45

  • The deprecation warning regarding asyncio.async() has been fixed. #51

0.5.0 (2016-09-07)

  • Introduced a changelog. #31

  • The event_loop fixture is again responsible for closing itself. This makes the fixture slightly harder to correctly override, but enables other fixtures to depend on it correctly. #30

  • Deal with the event loop policy by wrapping a special pytest hook, pytest_fixture_setup. This allows setting the policy before fixtures dependent on the event_loop fixture run, thus allowing them to take advantage of the forbid_global_loop parameter. As a consequence of this, we now depend on pytest 3.0. #29

0.4.1 (2016-06-01)

  • Fix a bug preventing the propagation of exceptions from the plugin. #25

0.4.0 (2016-05-30)

  • Make event_loop fixtures simpler to override by closing them in the plugin, instead of directly in the fixture. #21

  • Introduce the forbid_global_loop parameter. #21

0.3.0 (2015-12-19)

  • Support for Python 3.5 async/await syntax. #17

0.2.0 (2015-08-01)

  • unused_tcp_port_factory fixture. #10

0.1.1 (2015-04-23)

Initial release.


Contributions are very welcome. Tests can be run with tox, please ensure the coverage at least stays the same before you submit a pull request.

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